diff --git a/acorn/src/regexp.js b/acorn/src/regexp.js
index eaebceb14..65e5dc019 100644
--- a/acorn/src/regexp.js
+++ b/acorn/src/regexp.js
@@ -50,6 +50,7 @@ export class RegExpValidationState {
     this.maxBackReference = 0
     this.groupNames = Object.create(null)
     this.backReferenceNames = []
+    this.branchID = null
   }
 
   reset(start, pattern, flags) {